Localized deposits of silicon nitride, which are stable to at least 5000 C, have been formed by a new technique: electron bombardment of nitrogen molecules weakly bound on a clean Si(100)-(2 x 1) surface chilled to T ~ 30° K. This process is fairly efficient; for an initial coverage of one monolayer of molecular nitrogen, we estimate the effective dissociation -15 cross section (primary electron energy =2000 eV) to be (0.54 -1.2) x 10 2cm. Using Auger electron spectroscopy and LEED, we have studied the growth of a silicon nitride/silicon interface rigorously free from contamination and from damage due to sputtering or ion implantation. In the Si(LVV) Auger spectrum of silicon nitride, a strong peak at 83 eV predominates; the 9l-eV peak characteristic of clean Si vanishes entirely for sufficiently thick nitride films (~ 25 -30 A). LEED measurements, with the substrate at T ~ 30° K, reveal no ordered overlayers--the pattern stays (2 x 1), but the background increases with nitridation until a fully disordered structure results. Our Auger and LEED data further indicates that the initial stage of electron-induced nitridation is the formation of a monolayer of chemisorbed nitrogen via the nucleation and lateral growth of islands. Preliminary experiments have demonstrated that local deposits of silicon dioxide may be formed by the same technique used for nitridation: electronstimulated oxidation is more rapid with the substrate at T ~ 30° K than at room temperature. With proper outgassing of all vacuum components, particularly hot filaments, oxidation proceeds without the simultaneous growth of a surface carbon layer.